Currently, growth stimulants are used to enhance the body weight of animals which are raised for slaughtering, such as cattle, swine, turkeys, chickens, and the like. In the case of cattle and swine, approved growth hormones are administered as solid pellets which are injected into the ears of such animals. The ears are commonly discarded in slaughtering, such that no unabsorbed residues of such pellets will end up in food products intended for humans or domestic animals. The pellets are formulated for timed release and absorption of the active ingredients over an extended period of time.
The pellets are normally implanted while an animal is confined in a chute. An ear is grasped in one hand, and an implanter device having a large hypodermic needle is used to puncture the hide to enable a pellet dose to be injected between the hide and the next layer of tissue in the ear. The implanting must be done carefully to insure that the pellets are properly placed and that no pellet remains in the puncture in the hide, which could result in an infection. At the same time, the procedure must be carried out quickly since the animals are not entirely cooperative and may shake their heads to free the held ear. Further complicating the matter is that other procedures may be occurring at the same time as the implanting operation while the animal is confined, such as ear tagging, branding, veterinary inspections or procedures, or the like, which may further excite the animal.
The great majority of implanter devices employ manual gripping force on a trigger and a hand grip of such a device to propel an impeller through a pellet holding device or magazine to drive the pellets through the needle and into the space formed by the needle as the needle is withdrawn from the ear. Most implanters have a spring arrangement whereby an impeller return force is stored in the spring as the impeller is driven forward by operation of the trigger to return the impeller to its retracted position when the trigger is released. With such an arrangement, pellet implanting is complicated by the need to coordinate withdrawal of the needle as the pellets exit the needle. Such complexity of motion coupled with fatigue from using grip strength to eject the pellets can result in mistakes, such as lodging a pellet in the hide puncture or some of the pellets being ejected onto the ground.
U.S. Pat. No. 4,672,515 discloses an implanter which latches the trigger in the extreme extended position of the implanter and which provides a spring bias to the impeller in its extended position which causes the pellets to be automatically ejected as the needle is withdrawn from the ear of the animal. A release lever is operated to release the trigger latch after the needle is withdrawn to allow the impeller to return to its retracted position. Such an arrangement greatly increases the potential accuracy of implanting. However, fatigue can still be a factor since the grip strength of the person performing the implanting is used to propel the impeller against the force of the return spring arrangement.
A number of implanter devices use multiple pellet dose magazines to hold a plurality of pellet doses. Each pellet dose usually consists of a plurality of small pellets of a measured drug dosage which are positioned in an in-line orientation within a cylindrical chamber of the magazine. The magazine is a strip having a plurality of such chambers arranged in parallel relation, such as by being connected by webs between the chambers. Although some implanters are known to have magazines which advance to the next magazine chamber each time an implant operation occurs, most implanters require manual advancing of their magazines. Such manual advancing of the pellet magazine requires that the person performing the implanting operation remember to advance the magazine after each operation. If the magazine is not advanced, no pellets will be injected.